Modern reconstructive surgery was made possible by the stereo microscope. Early work showed that skin cannot simply be grafted—cut from one place and reattached in another—without attention to its blood supply. With no artery feeding it, it dies. Maintaining the old blood vessels while new ones grow from the attachment site places narrow constraints, on what donor and target sites can be combined.
This problem is solved by the transplant of more than skin: a surgical flap includes underlying tissue, and blood vessels which the surgeon joins to vessels in the target site. Since the vessels involved may be less than 1 mm in diameter, the accurate placement of six small sutures for a join where the blood flow without leaking requires magnification. This means more than merely an enlarged view. Dexterity in suturing requires depth perception, so that the needle can penetrate at a correct angle, including angles away from the viewer. The depth cue of parallax is unavailable through a microscope with a fixed viewpoint, perspective is unhelpful in a view with no straight lines and limited focal depth, and occlusion cannot show how far above the needle is above the tissue. It is essential to have the depth cue of stereopsis, with a lens system for each eye delivering views from different angles, to the two eyepieces. We refer to a pair of views that permit stereopsis as a stereo view or stereoscopic view, or as one having stereo. If the difference is correctly structured, the user's brain integrates the two views into a single scene with perceived depth, just as for direct vision with two eyes. Much surgery depends critically on this, as does dexterous work in other domains, such as industrial micro-assembly.
However, the surgical microscope requires that the user's head remain perfectly fixed, keeping the eyes to the eyepieces, throughout a long series of delicate procedures with substantial risk. This is an important source of stress on the surgeon, causing pain in the neck and back, and requiring several rest pauses per hour.
An alternative to enlarged display through optical lenses is to show on an electronic display the output of a real-time camera, digital or analogue. This technology is available, but in forms that fail to support stereo, that require the user to look away from the hands at a rotated view, or have both these problems. A view rotated from the natural direction requires the user to handle the fact that “to turn the instrument in the image this way, I must turn my hands that way,” adding to the cognitive difficulty, strain and learning curve of the task.